1. Field of the Invention
The present invention relates to a worm and a worm wheel to be used for a worm speed reducer, and also relates to a manufacturing method for such a worm and a worm wheel.
2. Description of the Prior Art
As shown in FIGS. 7 and 8, a known worm speed reducer is constituted of a worm 2 formed on a worm shaft 1 as an input shaft and a worm wheel 4 fixed to an output shaft 3. The worm 2 and the worm wheel 4 are in mesh with each other so as to perform rolling and sliding motion through tooth surfaces thereof and transmit power as well as the motion. In the worm speed reducer shown in FIGS. 7 and 8, rotation of the worm shaft 1 is reduced in speed to be transmitted to the output shaft 3.
In a worm speed reducer, even when a worm and a worm wheel are identical in dimension with other ones, they are very different in allowable transmitting horsepower and seizure resistance from the other ones according to a manner of combination of materials. In general, a problem occurring most frequently in a worm speed reducer to be operated continuously is seizure of tooth surfaces, so that seizure resistance is a greatly important factor.
As an example of a worm speed reducer improved in seizure resistance, it is known that surface-hardened alloy steel is employed for the material of a worm and it is ground with a high accuracy, while a relatively soft bronze material such as phosphor bronze or aluminum bronze is employed for the material of a worm wheel, in order to provide good fit between the worm and the worm wheel.
However, the precision grinding of the worm causes an increase in manufacturing cost, and the employment of the bronze material for the worm wheel causes an increase in material cost itself. Although a worm speed reducer can provide a large reduction ratio in one stage and employs a small number of parts, it becomes expensive to improve seizure resistance.
In contrast, as a low-cost worm speed reducer, it is known that an iron material is employed for both a worm wheel and a worm. For example, cast iron is employed for the material of the worm wheel, and cast iron or steel is employed for the worm. The worm is manufactured by cutting only without performing precision grinding.
However, in such a low-cost worm speed reducer, seizure resistance of the material itself is low, and it is difficult to finish a tooth surface of the worm with a high hardness and a high accuracy. For this reason, an allowable transmitting torque is low, and a lubricating condition in an initial stage of operation becomes severe in association with a machining error. Accordingly, sufficient break-in must be performed to obtain good fit between tooth surfaces of the worm and the worm wheel.
Further, it is also generally known to form a chemical conversion coating such as a phosphate coating on a contact surface (tooth surface) of a gear, thereby reducing a coefficient of friction of the contact surface owing to the chemical conversion coating. A coefficient of friction of the chemical conversion coating itself is not low, but the coefficient of friction of the contact surface is reduced by retaining a large amount of lubricating oil in a fine irregularity formed on the contact surface.
Accordingly, although it is considered that the above known chemical conversion coating is to be formed on a contact surface of a worm, since the chemical conversion coating itself is apt to wear, it will be peeled off in a short time. Thus, the effect of the chemical conversion coating will disappear during an initial low-load operation of break-in of a worm speed reducer, so that improvement in transmitting torque cannot be expected.
On the other hand, it is also known that a network groove having a width of about 20.mu. and a depth of about 30.mu. at a pitch of about 50.mu. is formed on a tooth surface (contact surface) of a relatively soft gear (worm wheel) of a pair of gears meshing with each other, thereby retaining a lubricating oil in the network groove to improve a lubricating ability during break-in of the gears (see Japanese Utility Model Publication No. 61-8278).
However, such a worm wheel is manufactured by preliminarily finely grinding the contact surface of the gear and then forming the network groove on the contact surface. Accordingly, a manufacturing step is increased to cause an increase in cost. Furthermore, since a soft material, e.g., a bronze material is required for the worm wheel, the cost is further increased from the viewpoint of the material.
Moreover, since the pitch of the network groove is large, a contact load strongly acts on a crest portion of the irregular contact surface to possibly increase wear of the contact surface. Further, since the network groove is constructed so as to become effective only in break-in and it will disappear by the time the break-in is ended, the improvement in the lubricating ability is temporary.
Additionally, since the network groove formed on the contact surface of the worm wheel is intended to improve the lubricating ability only during break-in, the irregularity on the contact surface due to the network groove will be worn off during a period of time such as by the time the break-in is ended. For this reason, a chemical conversion coating is not formed on the irregularity.
In general, meshing contact surfaces of a worm and a worm wheel of a worm speed reducer are required to have a high precision, so that they are finished by cutting or grinding. In case of requiring a low cost, the tooth surface of the worm is finished by cutting with a milling cutter 6 as shown in FIG. 6. Referring to FIG. 6, a contact surface 5 of the worm 2 formed on the worm shaft 1 is cut in a tooth trace direction X by the milling cutter 6. As a result, a cutting scratch consisting of a crest portion A and a trough portion B as shown in FIG. 1(a) is formed on the contact surface of the worm 2 in a tooth depth direction Y (perpendicular to the tooth trace direction X). The crest portion A and the trough portion B are substantially smooth in the tooth trace direction X as shown in FIG. 1(b).
Similarly, a cutting scratch consisting of a crest portion C and a trough portion D as shown in FIG. 1(c) is formed on a contact surface 7 of the worm wheel 4 in a tooth depth direction V (see FIG. 8). The crest portion C and the trough portion D are substantially smooth in a tooth trace direction U (see FIG. 8) as shown in FIG. 1(d).
As shown in FIGS. 2(e) to 2(h), in the case that a chemical conversion coating F such as a phosphate coating, an oxalate coating or a chromate coating is formed on the contact surface of the worm and the worm wheel shown in FIGS. 1(a) to 1(d), the chemical conversion coating F at the crest portion A of the worm meshing with the worm wheel is peeled off in a short time, and the chemical conversion coating F at the crest portion C of the worm wheel meshing with the worm is also peeled off in a short time.
Accordingly, no lubricating oil film is formed between the tooth surfaces of the worm and the worm wheel because of contact of the crest portions A and C. That is, the tooth surfaces contact each other in the condition of boundary lubrication or direct contact, causing the occurrence of seizure of the tooth surfaces. Therefore, a transmitting torque of the prior art worm speed reducer is limited by the seizure of the tooth surfaces.